1. Field of the Invention
The invention relates to resin extrusion apparatus and particularly to extrusion heads for producing articles with one or more tubular layers such as parisons, wire coatings, blown films, pipes, pultruded rods, profiles, reinforced sheets, etc.
2. Description of Prior Art
Parisons, tubular extrusions of plastic resin which are subsequently blow molded to form bottles or other containers, are formed in the prior art by various apparatus some including extrusion heads which extrude successive layers of plastic resin onto a mandrel. A typical multilayer parison extrusion head has separate inlets for receiving heated and plasticized resin from individual screw extruders and has separate channels for distributing and expressing the respective plastic resins in successive layers on the mandrel. Each channel includes an annular equalization and distribution chamber surrounding and spaced from the mandrel for receiving the plastic resin from the corresponding inlet. From the equalization chamber the plastic resin is fed though a frustoconical transfer passage downward and inward to a tubular extrusion channel formed around the mandrel. This annular extrusion channel exits through an inwardly or outwardly flaring annular die which includes a conical core member which may be moved longitudinally relative to the outer member of the die to vary the thickness of the wall of the extruded tubular article.
One particularly useful and successful extrusion head for forming tubular articles such as parisons is disclosed in U.S. Pat. No. 4,798,526. This head includes one or more individual annular extrusion modules surrounding respective successive portions of a stepped or tapered mandrel to form the annular extrusion channel which receives one or more successively extruded plastic resin layers from the modules. Each module has a pair of members with mating surfaces wherein the equalization and distribution chamber and the frustoconical transfer passage are formed. Having the head formed from individual coaxially spaced extrusion modules enables the head to be readily assembled and disassembled as well as enabling the assembly of an extrusion head with varying numbers of modules so that a module can be used in one assembled head to extrude a single layer tubular article or can be used in a differently assembled head to extrude any layer of a multilayer article. The modules forming an extrusion head can be positioned in any desired order. Furthermore, the die modules are separated from each other by annular air spaces. Concentric tubular necks or collars extend from each module into engagement with an adjacent module to define the separation distance or the width of the air spaces between modules. These air spaces prevent heat transfer from a high temperature module to an adjacent low temperature module. Low temperature resins can be degraded if heated to the higher temperature. This patent further discloses polymer inlet pressures at the extrusion head of 4,000 to 6,000 psi (27,000 to 41,000 KPa) for polycarbonate, 2,500 to 4,000 psi (17,000 to 28,000 KPa) for polypropylene, and 2,000 to 3,000 psi (13,000 to 21,000 KPa) for tie resins and barrier resins.
U.S. Pat. Nos. 3,649,143, 4,111,630 and 4,182,603 disclose tubular extrusion dies for blow molding of multi-ply films and having nested frustoconical, hemispherical and cylindrical die members forming polymer distribution chambers wherein spiral and helical grooves are formed on the outer surfaces of inner members. These grooves progress from points near or at the inlets toward the outlets with decreasing depth so that the polymer flow is gradually forced out of the grooves and into the frustoconical, hemispherical or tubular space between die members to evenly distribute the polymer around the chamber. These nested arrangements have several deficiencies such as limiting any temperature differential between the different layers being extruded, requiring larger heads for extruding greater numbers of layers, and having long conical passages from the end of the groove or grooves to the annular outlet.
Generally the prior art tubular extrusion apparatus requires a restrictive frustoconical transfer passage from the distribution chamber to the annular outlet from which the tubular article is extruded. This restrictive passage provides a relatively large pressure drop, i.e., greater than 50% of the total pressure drop from the extrusion head inlet through the distribution region or chamber and the restrictive passage to the outlet, in order to assist in even distribution of the polymer in the distribution chamber. In the absence of the restrictive outlet passage with the relatively large pressure drop, the polymer tends to flow at a greater rate along the shortest path between the inlet to the distribution chamber and the closest region of the annular outlet producing unevenness in the thickness of the tubular article about its circumference.
While the prior art apparatus is generally efficient and successful in the extrusion of tubular articles such as multilayer parisons, blown films, wire coatings, etc., there is room for improvement. In addition to the pressure differential problems discussed above, the stresses caused by temperature differentials need to be reduced. For instance, polymer melt in prior art extrusion heads flow from the inlet side of an equalization and distribution chamber to the opposite side. As a result, resins processed in prior art extrusion heads are subjected to temperature differentials, as well as subjected to the above-described pressure differentials, as the resin is processed through the inlet, the distribution means, and outlet. As a result of these processing differentials, the resin extrudate will deposit in a non-homogeneous distribution, thereby creating actual and latent internal stresses in the extruded parison. Then when the parison is formed into a final shape of a bottle or similar container, such internal stresses will be carried forward, and perhaps in some instances even exacerbated, in the final product. For instance, if the bottle or like article is to be reused and subjected to high temperature cleaning or the like and filled under pressure, e.g., with carbonated beverage, the article may explode as a result of such internal stresses. Given that prior extrusion apparatus are likely unable to alleviate these stresses there is a need to further improve extrusion heads. This is especially so in light of environmental demands which require reuse of containers rather than their disposal in landfills or the like.
Accordingly an object of the present invention is to construct a new and improved polymer extrusion head and process for tubular extrusions which minimize processing differentials that occur in prior art extrusion heads and processes and thereby resulting in a more homogenous distribution of polymer as it is extruded.
It is also an object of the invention to provide a container having superior properties as a result of the improved polymer distribution described above, and to thereby provide a container having sufficient properties to be effectively and safely reused or reclaimed.